Umm, the point of taking the backup tape home is to keep it off site. It is part of a contingency plan that all government agencies use. If the building is destroyed, a fireproof safe isn't going to help them keep running the very next day. It isn't the intern's fault for what happened beyond leaving it in his car, but at the same time, it is a shit job for someone to do, and it is always low man on the totem pole that takes the tapes home at night because they are forced to be first there and last to leave.
Although, I will say that whenever I bring my laptop home, it goes in my safe when I am not using it. SSN are nothing more than covered by the privacy act. The stuff I have is U.S. Gov't Secret.